请帮忙解释一下什么是自旋回波？

The NMR spin echo experiment
Sofar we have described the NMR FID experiment. We have seen that the essence of that experiment is the usage of a single r.f. pulse. However, an alternative way of measuring an NMR signal is the spin echo method. In that case a second r.f. pulse is given after a certain delay time , now with a related precession angle of . The effect of the second pulse is that at time a so-called nuclear spin echo can be detected [7]. This spin echo has an increasing character for and a decreasing character for (see Figure 1.11). In general the increasing or decreasing behaviour need not to be according to an exponential function, but for the sake of simplicity we will assume so in these lecture notes. In that case the lineshape function for the spin echo becomes




(1.25)



where , and in general .

In principle the NMR spin echo time-domain signal contains the same information concerning frequencies and line intensities as the NMR FID signal. However, an important reason for using the spin echo pulse sequence is that the delay time between the r.f. pulses offers the opportunity of manipulating the nuclear magnetization, for instance for creating spatial localization.



Figure 1.11: Real part of a complex-valued H spin echo time-domain signal of a human brain.

请问这个FID信号里边的一个回波，就是指的一个突起吗？？（好像一共有三个突起） 自旋回波脉冲序列中，X方向的90度脉冲会使磁化矢量偏向Y轴，这个电磁脉冲是如何作用的啊，为什么就向Y偏转呢？